Display apparatuses utilizing LCD and ELD have a step-up circuit for stepping up a given power source voltage to a drive voltage for driving the display. Expressive power of such display means have been increasingly improved in recent years. For example, display apparatuses of compact communications apparatuses such as PDAs and cellular phones have a high-resolution, varied gradation ability (ranging from monochrome to multi-color gradation), multi-color capability (ranging from monochrome to full colors). To drive LCD and ELD elements in a multi-color mode, it is necessary to drive them under an optimum driving condition. For this reason, it is necessary for a display apparatus to have means for accurately following the temperature dependent threshold voltage and the response time of the LCD and the ELD elements used to compensate for a change in temperature, correcting the drive conditions of these elements. In what follows an LCD element will be discussed as a typical example.
An LCD panel is conventionally equipped with a temperature sensor for providing digital temperature data of the display panel. The data can be used to control the display panel to be operated in the optimum condition (see the Japanese Patent Early Publication No. H5-273941).
In conventional LCD apparatuses, measurement of the temperature is performed as it is needed. Therefore, the temperature measurement is performed independently of the control of the step-up circuit.
Generally, a temperature sensor is adapted to detect temperature by detecting a minute change in voltage or current within a circuit. When the step-up circuit is stepping up a power source voltage towards a required step-up voltage through switching operation of switches (then the circuit is referred to as being in active state), it is likely that the switching operation generates a voltage noise on, for example, power supply lines, ground lines, and signal lines involved. Voltage noise also appears very often when a large drive current is passed to a display panel (that is, when the drive current changes rapidly). The voltage noise becomes large especially when a display apparatus has a step-up circuit, a temperature sensor, and a drive circuit all formed on the same semiconductor substrate. Since voltage noise becomes a source of error in the temperature measurement, accurate temperature compensation of the operating conditions cannot be attained in such display apparatus. It is possible to reduce the voltage noise by increasing the capacity of the power supply source or by increasing the thickness of power supply lines and ground lines. However, it is not a realistic solution, since it causes an increase of cost and increases the dimensions of the display.